Optical pickup device and optical disk device

ABSTRACT

An optical disk apparatus according to the present invention includes: a motor for rotating an optical disk; a light source; diffraction means for diffracting a portion of light emitted from the light source to form a main beam of 0 th  order light and a pair of sub beams composed of +1 st  order light and −1 st  order light which are formed on both sides of the 0 th  order light; an objective lens for converging the main beam and the pair of sub beams onto the optical disk; a light receiving means for receiving the main beam and the sub beams reflected from the optical disk, and outputting electrical signals through photoelectric conversion; and a calculation section for, based on the electrical signals output from the light receiving means, providing a main push-pull signal MPP, a sub push-pull signal SPP, and a differential signal between the main push-pull signal MPP and the sub push-pull signal SPP. A phase difference detection means for detecting a phase difference between the main push-pull signal MPP and the differential signal is further included, and in accordance with an output from the phase difference detection means, an offset is applied in a tracking control of the main beam with respect to the optical disk to compensate for an off-tracking caused by a phase shift of the differential signal.

TECHNICAL FIELD

The present invention relates to a pickup device which is capable of, byusing a laser light source, optically recording information (data) to aninformation storage medium such as an optical disk, or reproducinginformation (data) which is recorded on the information storage medium:and an optical disk apparatus comprising the pickup device.

BACKGROUND ART

In a drive apparatus (optical disk apparatus) which is capable ofperforming optical recording/reproduction of information for adisk-shaped optical disk, focusing control and tracking control arecarried out so as to cause the focal point of a light beam to be placedat a desired position on the recording surface of a rotating opticaldisk, by using a spindle motor or the like. In an optical disk apparatuswhich is capable of performing recording/reproduction of information foran optical disk such as a CD-R or a CD-RW, tracking control based on adifferential push-pull (Differential Push-Pull: DPP) technique isperformed. A DPP technique generates a tracking error signal by applyingcalculations to output signals from respective photodetectors, which areobtained from a main beam and two sub beams.

Hereinafter, with reference to FIG. 1, a DPP technique which isperformed in the aforementioned optical disk apparatus will bespecifically described. FIG. 1 shows the structure of an optical system10 in an optical pickup of an optical disk apparatus. In this opticalsystem 10, a diffraction grating 202 is disposed in a forward path of alight beam which is emitted from a laser light source 201. Thediffraction grating 202 diffracts the light beam which is emitted fromthe laser light source 201 to generate three beams of light, i.e.,0^(th) order diffracted light (main beam) and two beams of 1^(st) orderdiffracted light (sub beam). The above three beams of light which havebeen generated through diffraction by the diffraction grating 202 formthree light spots on an optical disk 206 via a beam splitter 203, acollimating lens 204, and an objective lens 205. Light which has beenreflected by the optical disk 206 is received by a photodetector 208 viathe beam splitter 203 and a detection lens 207.

Now, with reference to FIG. 8, relationship between the positions ofspots of the three beams of light formed on the optical disk 206 will bedescribed. FIG. 8(a) is a plan view schematically showing a relationshipof spot positions in a state where the optical disk 206 is not tiltedwith respect to the optical system 10 of FIG. 1. For reference's sake, apartial cross section of the optical disk is shown below the plan view.

As can be seen from FIG. 8(a), a spot of the main beam 30 is formed on apredetermined recording track among a plurality of recording tracks. Onboth sides of the recording track which is followed by the main beam 30,spots of the sub beams 32 and 33 are formed. More specifically, spots ofthe sub beams 32 and 33 are positioned near the centers of the guidetracks which are on both sides of the recording track on which the spotof the main beam 30 is positioned. As a result, the positions of thespots of the sub beams 32 and 33 on the optical disk, along the radialdirection, are shifted with respect to the position of the spot of themain beam 30 by ±0.5 track pitches.

FIG. 2 shows a detailed structure of the photodetector 208. As shown inFIG. 2, the photodetector 208 includes a main-beam photodetector 301which is irradiated with the main beam 30 having been reflected from theoptical disk 206, and sub-beam photodetectors 302 and 303 which arerespectively irradiated with the sub beams 33 and 32 having beenreflected from the optical disk 206. Through photoelectric conversion,the photodetector 208 outputs electrical signals which are in accordancewith the intensity of the light received by each detection section.

The main-beam photodetector 301 is split in four: detection sections 301a, 301 b, 301 c, and 301 d. The sub-beam photodetector 302 is split intwo: detection sections 302 e and 302 f. The sub-beam photodetector 303is split in two: detection sections 303 g and 303 h.

The split detection sections 301 a, 301 b, 301 c, 301 d, 302 e, 302 f,303 g, and 303 h output signals A, B, C, D, E, F, G, and H,respectively. By subjecting these signals A to H to calculations,tracking servo error signals are generated. Specifically, based on thesignals A to D which are output from the main-beam photodetector 301, anMPP calculation circuit 304 generates a main push-pull signal (MPP).Based on the signals E to H which are output from the respectivesub-beam photodetectors 302 and 303, an SPP calculation circuit 305generates a sub push-pull signal (SPP) and a DPP calculation circuit 306generates a differential push-pull signal (DPP).

The aforementioned calculations performed by the MPP calculation circuit304, the SPP calculation circuit 305, and the DPP calculation circuit306 are executed in accordance with (eq. 1), (eq. 2), and (eq. 3) shownbelow.MPP=(A+D)−(B+C)  (eq. 1)SPP=SPP1+SPP2=(F−E)+(H−G)  (eq. 2) $\begin{matrix}\begin{matrix}{{DPP} = {{MPP} - {\alpha \times {SPP}}}} \\{= {\left( {A + D} \right) - \left( {B + C} \right) - {\alpha \times \left\{ {\left( {F - E} \right) + \left( {H - G} \right)} \right\}}}}\end{matrix} & \left( {{eq}.\quad 3} \right)\end{matrix}$

Herein, α is a constant which is determined based on the intensities ofthe 0^(th) order diffracted light, +1^(st) order diffracted light, and−1^(st) order diffracted light. Although eq. 3 includes the coefficienta, the differential push-pull signal (DPP) is, in the broad sense of theword, a differential signal between the main push-pull signal (MPP) andthe sub push-pull signal (SPP).

According to the aforementioned tracking servo method, as shown in FIG.2, the positioning of optical elements such as the diffraction grating202, the laser light source 201, the photodetector 208 is set so thatthe respective beams will be positioned on the centers of the splitlines of the photodetectors 301, 302, and 303.

FIG. 3 shows signal waveforms 401, 402, and 403 of the main push-pullsignal (MPP), the sub push-pull signal (SPP), and the differentialpush-pull signal (DPP) in the case where the aforementioned idealpositioning is realized.

As is clear from FIG. 3, the phase of the SPP waveform 402 is shifted byπ rad (180°) with respect to the phase of the MPP waveform 401, the twowaveforms being of inverted relationship. Such a relationship isobtained because, as shown in FIG. 8(a), the spots of the sub beams 32and 33 are positioned not on recording tracks but on guide tracks, sothat their signal polarities are inverted.

Since the polarity of the SPP waveform 402 and the polarity of the MPPwaveform 401 are opposite, the phase of the DPP waveform 403 which isobtained in accordance with eq. (3) has the same phase as that of theMPP waveform 401.

In the case where the optical disk 206 is not tilted, as shown in FIG.8(a), the light spot of the main beam 30 upon the optical disk 206 is ona track center at the position indicated by reference numeral “40” inFIG. 3. The DPP waveform 403 is calibrated and set so as to indicate azero value at this time.

In the DPP technique, there is performed a tracking control for causingthe entire objective lens or the entire optical pickup device to movealong a radial direction of the optical disk 206 in such a manner thatthe DPP waveform 403 exhibits a zero value. Since the light spot that isthe target of the tracking control is the light spot of the main beam,the light spot of the main beam will be abbreviated as the “light spot”in the following descriptions, for simplicity.

The above-described conventional optical pickup device is disclosed in,for example, Japanese Laid-Open Patent Publication No. 2001-307351.

In the case where the optical disk 206 or the objective lens 205 istilted along a radial direction of the optical disk, the MPP, SPP, andDPP signal waveforms change to an MPP waveform 501, an SPP waveform 502,and a DPP waveform 503, respectively, as shown in FIG. 4. This isbecause, if the optical disk 206 is tilted as shown in FIG. 8(b), themain beam 30 and the sub beams 32 and 33 will be obliquely incident on arecording track/guide tracks on the optical disk 206. As a result, aphase difference occurs between the MPP waveform 501 and the SPPwaveform 502. Assume that a phase difference of φ emerges between theMPP waveform 501 and the SPP waveform 502. In this case, the phase ofthe DPP waveform 503 is shifted from the phase of the ideal signalwaveform, which would exhibit a zero value when the light spot is on atrack center, and has a phase difference with a magnitude of “φ”.Therefore, when a tracking control is performed based on such a DPPwaveform 503, the DPP waveform 503 will indicate a zero value at aposition indicated by reference numeral “51” in FIG. 4, and thereforethe actual light spot will be controlled to a position which is shiftedby a distance Δ, which corresponds to the phase difference φ, from thetrack center (i.e., a position indicated by reference numeral “50”).This distance Δ will be referred to as an “off-tracking amount” of thelight spot position. Although the off-tracking amount Δ is described inrelation to the DPP waveform 503 in FIG. 4, the actual off-trackingamount is a distance between the light spot position of the main beamand a recording track center on the optical disk.

With the DPP signal (tracking error signal) which has incurred a phaseshift as above, it is impossible to control the position of the lightspot to be accurately on a track center, so that the tracking controlbecomes unstable. This results in an off-tracking, i.e., deviation of alight spot on an optical disk from a track center, whereby therecording/reproduction characteristics of the optical disk apparatus aredeteriorated.

The present invention has been made in order to solve the aboveproblems, and is aimed at providing: an optical pickup device capable ofcorrecting an off-tracking which is ascribable to a phase shift of a DPPsignal waveform, even when an optical disk or an objective lens istilted along a radial direction of the optical disk, so that stabletracking control can be performed; and, an optical disk apparatuscomprising such an optical pickup device.

DISCLOSURE OF INVENTION

An optical disk apparatus according to the present invention comprises:a motor for rotating an optical disk; a light source; diffraction meansfor diffracting a portion of light emitted from the light source to forma main beam of 0^(th) order light and a pair of sub beams composed of+1^(st) order light and −1^(st) order light which are formed on bothsides of the 0^(th) order light; an objective lens for converging themain beam and the pair of sub beams onto the optical disk; lightreceiving means for receiving the main beam and the sub beams reflectedfrom the optical disk, and outputting electrical signals throughphotoelectric conversion; a calculation section for, based on theelectrical signals output from the light receiving means, providing amain push-pull signal MPP, a sub push-pull signal SPP, and adifferential signal between the main push-pull signal MPP and the subpush-pull signal SPP; and phase difference detection means for detectinga phase difference between the main push-pull signal MPP and thedifferential signal, wherein, in accordance with an output from thephase difference detection means, an offset is applied in a trackingcontrol of the main beam with respect to the optical disk to compensatefor an off-tracking caused by a phase shift of the differential signal.

In a preferred embodiment, the differential signal is a differentialpush-pull signal DPP.

In a preferred embodiment, the light receiving means comprises: amain-beam photodetector having four split photoelectric conversionsections for receiving the main beam reflected from the optical disk; afirst sub-beam photodetector having two split photoelectric conversionsections for receiving one of the pair of sub beams; and a secondsub-beam photodetector having two split photoelectric conversionsections for receiving the other of the pair of sub beams, and thecalculation section further comprises: first calculation means fordetermining the main push-pull signal MPP=(A+D)−(B+C), based on signalsA, B, C, and D obtained respectively from the four split photoelectricconversion sections of the main-beam photodetector; second calculationmeans for determining the sub push-pull signal SPP=(F−E)+(H−G), based onsignals E and F obtained respectively from the two split photoelectricconversion sections of the first sub-beam photodetector and on signals Gand H obtained respectively from the two split photoelectric conversionsections of the second sub-beam photodetector; and third calculationmeans for determining the differential push-pull signal DPP=MPP−α×SPP(where α is a constant), based on outputs from the first calculationmeans and the second calculation means.

In a preferred embodiment, signal amplitude calculation means foradjusting amplitudes of the main push-pull signal MPP and/or the subpush-pull signal SPP so that the amplitude of the main push-pull signalMPP and the amplitude of the sub push-pull signal SPP become equal;signal summation means for calculating a sum of the main push-pullsignal MPP and the sub push-pull signal SPP which are output from thesignal amplitude calculation means; and phase difference calculationmeans for, based on an output from the signal summation means,calculating a phase difference between the main push-pull signal MPP andthe sub push-pull signal SPP are comprised.

An optical pickup device according to the present invention comprises: alight source; diffraction means for diffracting a portion of lightemitted from the light source to form a main beam of 0^(th) order lightand a pair of sub beams composed of +1^(st) order light and −1^(st)order light which are formed on both sides of the 0^(th) order light; anobjective lens for converging the main beam and the pair of sub beamsonto the optical disk; light receiving means for receiving the main beamand the sub beams reflected from the optical disk, and outputtingelectrical signals through photoelectric conversion; a calculationsection for, based on the electrical signals output from the lightreceiving means, providing a main push-pull signal MPP, a sub push-pullsignal SPP, and a differential signal between the main push-pull signalMPP and the sub push-pull signal SPP; and phase difference detectionmeans for detecting a phase difference between the main push-pull signalMPP and the sub push-pull signal SPP, wherein, in accordance with anoutput from the phase difference detection means, an offset is appliedin a tracking control of the main beam with respect to the optical diskto compensate for an off-tracking caused by a phase shift of thedifferential signal.

A driving method for an optical disk according to the present inventioncomprises: a step of converging a main beam and a pair of sub beams ontoan optical disk and outputting electrical signals based on the main beamand the sub beams reflected from the optical disk; a step of, based onthe electrical signals, providing a main push-pull signal MPP, a subpush-pull signal SPP, and a differential signal between the mainpush-pull signal MPP and the sub push-pull signal SPP; and a step ofdetecting a phase difference between the main push-pull signal MPP andthe differential signal, wherein, based on the phase difference, anoffset is applied in a tracking control of the main beam with respect tothe optical disk to compensate for an off-tracking caused by a phaseshift of the differential signal.

In a preferred embodiment, the differential signal is a differentialpush-pull signal DPP.

In a preferred embodiment, the step of providing the differential signalcomprises: a step of determining the main push-pull signalMPP=(A+D)−(B+C), based on signals A, B, C, and D obtained respectivelyfrom four split photoelectric conversion sections of a main-beamphotodetector; a step of determining the sub push-pull signalSPP=(F−E)+(H−G), based on signals E and F obtained respectively from twosplit photoelectric conversion sections of a first sub-beamphotodetector and on signals G and H obtained respectively from twosplit photoelectric conversion sections of a second sub-beamphotodetector; and a step of determining the differential push-pullsignal DPP=MPP−α×Spp (where α is a constant).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the structure of an optical system used inan optical pickup device.

FIG. 2 is a diagram showing the structure of a photodetector used in anoptical pickup device.

FIG. 3 is a diagram showing signal waveforms obtained by a DPPtechnique.

FIG. 4 is a diagram showing an MPP waveform and an SPP waveform having aphase difference, and a DPP waveform.

FIG. 5 is a diagram showing the structure of an optical pickup device(first embodiment) of an optical disk apparatus according to the presentinvention.

FIG. 6 is a diagram showing a DPP waveform having a phase shift, and aDPP waveform which has been subjected to an off-tracking correction.

FIG. 7(a) is a diagram showing the structure of a phase differencedetection circuit in an optical pickup device (second embodiment) of anoptical disk apparatus according to the present invention. FIG. 7(b) isa diagram showing the waveforms of signals which are generated throughcalculations in the second embodiment.

FIGS. 8(a) and (b) are diagrams schematically showing a positioningrelationship between light spots of three beams 30, 32, and 33 on anoptical disk.

FIG. 9 is a diagram showing the general structure of an optical diskapparatus according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the figures.

EMBODIMENT 1

First, FIG. 9 is referred to. FIG. 9 is a diagram showing the generalstructure of an optical disk apparatus according to the presentembodiment.

The illustrated optical disk apparatus comprises: a spindle motor 902for rotating an optical disk 206; an optical pickup device 904 whichoptically accesses a desired track on the optical disk 206; and a servosystem 906 for controlling the number of revolutions of the spindlemotor 902 and the position of the pickup device 904. Moreover, thisoptical disk apparatus comprises a signal processing section 908 forprocessing signals which are output from the optical pickup device 904,as well as a video decoder 910 and an audio decoder 912 for decoding avideo signal and an audio signal, respectively, which are output fromthe signal processing section 908. The specific structures of the signalprocessing section 908, the video decoder 910, and the audio decoder 912are identical to their known structures.

Although FIG. 9 illustrate constituent elements which are necessary fora reproduction operation of reading data which is recorded on theoptical disk 206, constituent elements (not shown) for recording dataonto the optical disk 206 may be comprised.

Next, FIG. 5 is referred to. FIG. 5 shows the structure of the opticalpickup device 904 in the present embodiment.

The optical pickup device 904 shown in FIG. 5 comprises an opticalsystem 10, a phase difference detection circuit 101, an off-trackingcorrection amount calculation circuit 102, and an objective lens drivingcircuit 103. The optical system 10 of the present embodiment is similarto the conventional optical system shown in FIG. 1, although anobjective lens driving device 104 not shown in FIG. 1 is shown in FIG.5. The objective lens driving device 104 drives the objective lens 205along a radial direction of the optical disk 206. The structure andoperation of the optical system 10 are similar to those of the opticalsystem 10 shown in FIG. 1, and the structure of the photodetector 208 isalso similar to the structure shown in FIG. 2, and therefore thedetailed descriptions of their structures and operations will not berepeated here.

In the present embodiment, in order to eliminate the aforementionedoff-tracking, the optical pickup device 904 comprises the phasedifference detection circuit 101 and the off-tracking correction amountcalculation circuit 102, and the objective lens driving circuit 103controls the driving of the objective lens 205 in an appropriate manner.

Hereinafter, with reference to FIG. 5, the phase difference detectioncircuit 101, the off-tracking correction amount calculation circuit 102,and the objective lens driving circuit 103 will be described.

The phase difference detection circuit 101 of the present embodimentfunctions as a phase difference detection means for detecting adifference in phase between the waveform of an MPP signal and thewaveform of an SPP signal obtained from the photodetector 208 in theoptical system 10 of the optical pickup device 904. The phase differencedetection circuit 101 simultaneously monitors the MPP signal waveformand the SPP signal waveform, and compares them to detect a phasedifference therebetween. The phase difference detection circuit 101 assuch can be realized by providing, for example, a circuit which monitorsthe time at which the output of each of the MPP signal and the SPPsignal becomes zero and detects a difference between such times, and acircuit which monitors the frequencies of the MPP signal waveform andthe SPP signal waveform and calculates the amount of phase differencecorresponding to this time difference. Such a circuit can be realized byhardware, software, or a combination of hardware and software. Forexample, since a servo processor (IC chip) incorporated in a DVD playeror recorder has a function of measuring the phases of signal waveforms,the phase difference detection circuit 101 can be realized by utilizingat least a portion of this servo processor.

The phase difference detection circuit 101 outputs a signal representingthe detected phase difference, and sends the signal to the off-trackingcorrection amount calculation circuit 102.

On the basis of the “phase difference” detected by the phase differencedetection circuit 101, the off-tracking correction amount calculationcircuit 102 calculates the amount of any off-tracking that occurs, andoutputs a signal which represents a off-tracking amount for correction.This signal is sent to the objective lens driving circuit 103. Theaforementioned “phase difference” is a phase difference between theaforementioned MPP signal and SPP signal, and occurs when the opticaldisk or the objective lens is tilted along a radial direction of theoptical disk, for example. A method for calculating an off-trackingamount based on a detected “phase difference” will be described later.

Based on the signal which is output from the off-tracking correctionamount calculation circuit 102, the objective lens driving circuit 103is able to drive the objective lens driving device 104 so as to move theobjective lens 205 along a radial direction of the optical disk.

Thus, in the present embodiment, in accordance with a signalrepresenting a phase difference which is output from the phasedifference detection circuit 101, an offset is applied in the trackingcontrol of the objective lens 205, whereby any off-tracking caused by aphase shift of the DPP signal as shown in FIG. 4 is compensated for.

Hereinafter, the correction operation for tracking control will bedescribed more specifically.

The off-tracking amount correction calculation circuit 102 shown in FIG.5 receives from the phase difference detection circuit 101 a signalrepresenting a phase difference (=phase shift amount of the DPP signal)φ detected between the MPP signal waveform 501 and the SPP signalwaveform 502 as shown in FIG. 4, and then calculates an off-trackingamount Δ according to the following equation.Δ=T×φ/2π  (eq. 4)

Herein, T is a track pitch of the optical disk.

The off-tracking amount correction calculation circuit 102 outputs asignal representing the off-tracking amount Δ which has been calculatedbased on (eq. 4), and sends the signal to the objective lens drivingcircuit 103 in FIG. 5.

The objective lens driving circuit 103 drives the objective lens drivingdevice 104 to move the objective lens 205 along a radial direction ofthe optical disk. At this time, the determination as to whether thedirection of the movement should be toward the outer periphery or theinner periphery of the optical disk is to be made so that theoff-tracking will be cancelled. Specifically, a direction of movementfor reducing an off-tracking amount is previously determined inaccordance with whether the MPP signal being output during trackingcontrol is positive or negative in the DC level. The direction ofmovement is to be determined in accordance with whether the MPP signalis positive or negative in the DC level.

Next, with reference to FIG. 6, the magnitude of off-tracking amountwill be described.

FIG. 6 shows a DPP signal waveform 601 in an ideal state where theoptical disk or the objective lens is not tilted along a radialdirection of the optical disk, and the DPP signal waveform 503 in astate where the optical disk or the objective lens is tilted along aradial direction of the optical disk.

In the case where a phase difference φ has occurred due to a tilt of theoptical disk or the like as in the DPP signal waveform 503, if theconventional tracking control is performed as it is, the position of thelight beam spot will be controlled to a position corresponding to apoint X, at which the DPP signal waveform 503 has a zero-cross. As aresult, the light spot is controlled to a position 61 which is shifted(off-tracked) from a track center. However, in the present embodiment,the off-tracking amount Δ is corrected so that a tracking control isperformed so as to retain the light beam spot at a position 60corresponding to a point Y of the DPP signal waveform 503.

The reason why the off-tracking amount Δ corresponding to the phasedifference φ is expressed as in eq. 4 is that Δ/T=φ/2π holds true when atrack pitch corresponding to the radial direction period of the DPPsignal waveform 503, 601 is T, as shown in FIG. 6.

Assume that the phase difference detected by the phase differencedetection circuit 101 has a magnitude which is represented as φ=π/4(radian), for example. At this time, if the optical disk to be subjectedto recording or reproduction is a DVD-R, the track pitch is T=0.74(μm),so that the off-tracking amount Δ calculated from (eq. 4) is(0.74×π/4)/2π=0.0925(μm).

In this case, the phase difference detection circuit 101 determines thedirection of corrected off-tracking based on whether the aforementionedphase difference is positive or negative, and outputs to the objectivelens driving circuit 103 a signal representing the aforementionedcorrected off-track amount and correction direction. Based on thissignal, the objective lens driving circuit 103 drives the objective lensdriving device 104 to move the objective lens 205 in the properdirection, along a radial direction of the optical disk, by thecorrected off-tracking amount. In this manner, according to the presentembodiment, even if a phase difference occurs between the MPP signal andthe SPP signal because the optical disk 206 or the objective lens 205has tilted along a radial direction of the optical disk, for example,the phase shift of the DPP signal which may occur responsive to thisphase difference can be compensated for. As a result, a stable trackingcontrol is enabled, and the recording/reproduction performance of theoptical pickup device 904 can be improved.

Instead of adopting the above-described structure according to thepresent embodiment, a DPP phase calculation means which corrects a phaseshift in the DPP signal in accordance with the output from the phasedifference detection circuit 101 may be used. In this case, the DPPphase calculation means applies an offset of phase φ, in the directionof an arrow 62, to the DPP signal waveform 503 shown in FIG. 6. As aresult, an ideal DPP signal waveform 601 which is free of phase shiftcan be output. In this case, since a usual tracking control centeredwhich is around a point Z (at which the DPP signal waveform 601 has azero-cross) is performed, the recording/reproduction performance of theoptical pickup device can be improved with a simple circuit structure.Note that the offset for such compensation is to be varied in accordancewith the measured phase shift , rather than being a constant value.

Moreover, the light receiving means in the present embodiment iscomposed of a four-split photodetector as the main-beam photodetectorand two-split photodetectors as the sub-beam photodetectors. However,the light receiving means according to the present invention is notlimited to such photodetectors. For example, the number of splits ineach sub-beam photodetector may be increased from two to four. In thatcase, it would be possible to generate a focus error signal by using thesignals which are output from the two sub-beam photodetectors.

Moreover, the present invention can be broadly applied to an opticaldisk apparatus which utilizes a differential push-pull (DPP) techniqueas a tracking method. The specific structure of the photodetector, e.g.,the number of splits and the manner of splitting, is not limited to thatin the present embodiment. Furthermore, applications to an optical diskapparatus utilizing a three-beam technique would also be possible.

In the present embodiment, the objective lens 205 (FIG. 5) is driven forperforming tracking control. Instead, the optical pickup device itselfmay be moved along a radial direction of the optical disk 206, by usinga mechanism for driving the optical pickup device 904. Also, thestructure of the optical system 10 of the optical pickup is not limitedto that which is shown in FIG. 1. Based on the outputs from thephotodetector 208, calculations similar to the calculations performed bythe MPP calculation circuit 304, the SPP calculation circuit 305, andthe DPP calculation circuit 306 shown in FIG. 2 may be performedexternally to the photodetector 208 to generate an MPP signal, an SPPsignal, and a DPP signal.

EMBODIMENT 2

Next, with reference to FIGS. 7(a) and (b), a second embodiment of theoptical disk apparatus according to the present invention will bedescribed. FIG. 7(a) shows an exemplary internal structure of the phasedifference detection circuit 101. Other than the phase differencedetection circuit 101, the structure of the optical pickup device of thepresent embodiment is identical to the structure of the optical pickupdevice 904 of Embodiment 1.

The phase difference detection circuit 101 of the present embodimentcomprises a signal amplitude calculation circuit 701, a signal summationcircuit 702, and a phase difference calculation circuit 703.

The signal amplitude calculation circuit 701 performs calculations(conversion) for an MPP signal and/or an SPP signal so that theamplitudes of the MPP signal waveform and the SPP signal waveform becomeelectrically equal to each other, and outputs the MPP signal and the SPPsignal whose amplitudes have become equal.

The signal summation circuit 702 receives the MPP signal and the SPPsignal which are output from the signal amplitude calculation circuit701, calculates a sum of both signals, and outputs the sum as a sumsignal SumPP. The phase difference calculation circuit 703 receives thesum signal SumPP which is output from the signal summation circuit 702,and based on the amplitude of the sum signal SumPP, determines the phasedifference between the MPP signal and the SPP signal according to aspecific calculation formula.

Referring to FIG. 7(b), a specific procedure of phase differencedetection will be described. FIG. 7(b) shows waveforms 704, 705, and 706of the MPP signal, the SPP signal, and the sum signal SumPP,respectively, generated by the above calculation circuits.

The MPP signal and SPP signal which are output from the DPP calculationcircuit 306 shown in FIG. 2 are first input to the signal amplitudecalculation circuit 701. Then, the signal amplitude calculation circuit701 outputs an MPP signal and an SPP signal which have been converted soas to have electrically equal amplitudes. Herein, by using the phasedifference φ occurring between the MPP signal and the SPP signal, timet(s), and angular velocity ω (1/s) of the signal, the MPP signal and theSPP signal can be expressed by the following equations.MPP=A·sin(ω·t)  (eq. 5)SPP=−A·sin(ω·t−φ)  (eq. 6)

Herein, A is a constant representing amplitude.

The MPP signal and SPP signal after conversion are input to the signalsummation circuit 702, and the signal summation circuit 702 calculates asum signal SumPP obtained by adding the MPP signal and the SPP signal,and outputs the result of the calculation to the phase differencecalculation circuit 703.

The sum signal SumPP is expressed by (eq. 7) below, based on (eq. 5) and(eq. 6).SumPP=MPP+SPP=A·sin(ω·t)−A·sin(ω·t−φ)=A·(2−2 cosφ)^(1/2)·sin(ω·t+δ)  (eq. 7)

Herein, δ is a constant representing phase.

From (eq. 7), the amplitude of the signal waveform of the sum signalSumPP which is input to the signal summation circuit 703 is a functionof φ as expressed by A·(2-2 cos φ)^(1/2).

Therefore, by detecting the amplitude of the signal waveform of the sumsignal SumPP which is input to the phase difference calculation circuit703, and subjecting it to calculations, the phase difference φ can beobtained. In other words, in the phase difference calculation circuit703, an amplitude B of the signal waveform of the SumPP which ismonitored can be expressed by (eq. 8) below.B=A·(2-2 cos φ)^(1/2)   (eq. 8)

As is clear from (eq. 8), by monitoring the values (magnitude) of theamplitude A and amplitude B of the respective signal waveforms, thevalue of the phase difference φ is obtained. A signal (which may be ofan arbitrary format, e.g., analog or digital) representing the value ofthe phase difference φ calculated in the above manner is sent to theoffset correction amount calculation circuit 102 in FIG. 5, and acorrection of the offset amount is performed. Specifically, in a mannersimilar to the operation which has been described with respect to theoptical pickup device of Embodiment 1, the off-tracking amountcorrection calculation circuit 102 calculates a corrected off-trackamount according to (eq. 4), and sends a signal (which may be of anarbitrary format, e.g., analog or digital) representing thisoff-tracking amount to the objective lens driving circuit 103. Based onthis signal, the objective lens driving circuit 103 drives the objectivelens driving device 104, moves the objective lens 205 along a diskradial direction, and corrects the off-tracking.

Thus, in accordance with the structure of the present embodiment, ifthere occurs an off-tracking that is ascribable to a phase difference φbetween the MPP signal and the SPP signal, which would occur when theoptical disk or the objective lens is tilted along a radial direction ofthe optical disk, the phase difference φ is detected, and a trackingcontrol is performed which is corrected in such a manner as to cancel anoff-tracking amount which is calculated by using the detected phasedifference φ. As a result, a stable tracking control is enabled, and therecording/reproduction performance of the optical pickup device can beimproved.

The calculations of the MPP signal and the SPP signal performed in orderto calculate the aforementioned phase difference φ can be easilyperformed by using calculation circuits comprised in an ordinary opticalpickup device (calculation circuits for the processing of signals froman optical disk, servo control, and the like). Thus, without having tonewly provide a phase detection circuit for the signal waveforms, theabove can be realized with a simple circuit structure, and a stabletracking control can be attained at a low cost.

The angle of a tilt of an optical disk along a radial direction as shownin FIG. 8(b) changes in accordance with the positioning relationshipbetween the optical disk and the optical pickup. Moreover, the tiltangle may rapidly fluctuate in a periodic manner while the optical diskis rotated by a motor. According to the present invention, in responseto such dynamic fluctuations of the tilt angle, the offset to be appliedto tracking control can be rapidly changed, whereby off-tracking can bereduced in a dynamic and adaptive manner. The reason why such dynamiccompensation is possible is that the optical disk apparatus of thepresent invention compensates for phase shifts in signal waveformsthrough calculations.

Note that, in the case where the changes in phase difference φ asdetected in the present invention are periodic, once the period isascertained, then there is no need to always perform the operation ofmeasuring the phase difference φ in real time. The periodically-changingmagnitude may be predicted to compensate for off-tracking. By doing so,it will be possible to reduce the calculation amount.

Furthermore, although the calculation processes which are necessary foroff-tracking compensation are performed inside the optical pickup devicein the above embodiments, a part or whole of such calculations may beperformed in any section in the optical disk apparatus other than theoptical pickup device (e.g., a calculation section in an IC chip such asa servo processor). Such a servo processor is provided in, for example,the servo system 906 shown in FIG. 9.

In accordance with the optical disk apparatus of the present inventionhaving the above structure, during data recording/reproductionoperations, a main beam and a pair of sub beams are converged onto anoptical disk, and based on the main beam and the sub beams reflectedfrom the optical disk, a step of determining a main push-pull signalMPP, a sub push-pull signal SPP, and a differential push-pull signal DPPis executed. Then, a step of detecting a phase difference between themain push-pull signal MPP and the differential push-pull signal DPP isexecuted. Furthermore, based on this phase difference, an offset isapplied in the tracking control of the main beam with respect to theoptical disk, thus compensating for any off-tracking caused by a phaseshift of the differential push-pull signal DPP.

INDUSTRIAL APPLICABILITY

According to the present invention, even in the case where an opticaldisk or an objective lens tilts along a radial direction of an opticaldisk, any off-tracking occurring due to a phase difference between anMPP signal and an SPP signal can be corrected, thus enabling a stabletracking control. As a result, the recording/reproduction performance ofan optical pickup device can be improved.

1. An optical disk apparatus comprising: a motor for rotating an opticaldisk; a light source; diffraction means for diffracting a portion oflight emitted from the light source to form a main beam of 0^(th) orderlight and a pair of sub beams composed of +1^(st) order light and−1^(st) order light which are formed on both sides of the 0^(th) orderlight; an objective lens for converging the main beam and the pair ofsub beams onto the optical disk; light receiving means for receiving themain beam and the sub beams reflected from the optical disk, andoutputting electrical signals through photoelectric conversion; acalculation section for, based on the electrical signals output from thelight receiving means, providing a main push-pull signal MPP, a subpush-pull signal SPP, and a differential signal between the mainpush-pull signal MPP and the sub push-pull signal SPP; and phasedifference detection means for detecting a phase difference between themain push-pull signal MPP and the differential signal, wherein, inaccordance with an output from the phase difference detection means, anoffset is applied in a tracking control of the main beam with respect tothe optical disk to compensate for an off-tracking caused by a phaseshift of the differential signal.
 2. The optical disk apparatus of claim1, wherein the differential signal is a differential push-pull signalDPP.
 3. The optical disk apparatus of claim 2, wherein the lightreceiving means comprises: a main-beam photodetector having four splitphotoelectric conversion sections for receiving the main beam reflectedfrom the optical disk; a first sub-beam photodetector having two splitphotoelectric conversion sections for receiving one of the pair of subbeams; and a second sub-beam photodetector having two splitphotoelectric conversion sections for receiving the other of the pair ofsub beams, and the calculation section further comprises: firstcalculation means for determining the main push-pull signalMPP=(A+D)−(B+C), based on signals A, B, C, and D obtained respectivelyfrom the four split photoelectric conversion sections of the main-beamphotodetector; second calculation means for determining the subpush-pull signal SPP=(F−E)+(H−G), based on signals E and F obtainedrespectively from the two split photoelectric conversion sections of thefirst sub-beam photodetector and on signals G and H obtainedrespectively from the two split photoelectric conversion sections of thesecond sub-beam photodetector; and third calculation means fordetermining the differential push-pull signal DPP=MPP−α×SPP (where α isa constant), based on outputs from the first calculation means and thesecond calculation means.
 4. The optical disk apparatus of claim 1,comprising: signal amplitude calculation means for adjusting amplitudesof the main push-pull signal MPP and/or the sub push-pull signal SPP sothat the amplitude of the main push-pull signal MPP and the amplitude ofthe sub push-pull signal SPP become equal; signal summation means forcalculating a sum of the main push-pull signal MPP and the sub push-pullsignal SPP which are output from the signal amplitude calculation means;and phase difference calculation means for, based on an output from thesignal summation means, calculating a phase difference between the mainpush-pull signal MPP and the sub push-pull signal SPP.
 5. An opticalpickup device comprising: a light source; diffraction means fordiffracting a portion of light emitted from the light source to form amain beam of 0^(th) order light and a pair of sub beams composed of+1^(st) order light and −1^(st) order light which are formed on bothsides of the 0^(th) order light; an objective lens for converging themain beam and the pair of sub beams onto the optical disk; lightreceiving means for receiving the main beam and the sub beams reflectedfrom the optical disk, and outputting electrical signals throughphotoelectric conversion; a calculation section for, based on theelectrical signals output from the light receiving means, providing amain push-pull-signal MPP, a sub push-pull signal SPP, and adifferential signal between the main push-pull signal MPP and the subpush-pull signal SPP; and phase difference detection means for detectinga phase difference between the main push-pull signal MPP and the subpush-pull signal SPP, wherein, in accordance with an output from thephase difference detection means, an offset is applied in a trackingcontrol of the main beam with respect to the optical disk to compensatefor an off-tracking caused by a phase shift of the differential signal.6. A driving method for an optical disk, comprising: a step ofconverging a main beam and a pair of sub beams onto an optical disk andoutputting electrical signals based on the main beam and the sub beamsreflected from the optical disk; a step of, based on the electricalsignals, providing a main push-pull signal MPP, a sub push-pull signalSPP, and a differential signal between the main push-pull signal MPP andthe sub push-pull signal SPP; and a step of detecting a phase differencebetween the main push-pull signal MPP and the differential signal,wherein, based on the phase difference, an offset is applied in atracking control of the main beam with respect to the optical disk tocompensate for an off-tracking caused by a phase shift of thedifferential signal.
 7. The driving method for a disk of claim 6,wherein the differential signal is a differential push-pull signal DPP.8. The driving method for a disk of claim 6, wherein the step ofproviding the differential signal comprises: a step of determining themain push-pull signal MPP=(A+D)−(B+C), based on signals A, B, C, and Dobtained respectively from four split photoelectric conversion sectionsof a main-beam photodetector; a step of determining the sub push-pullsignal SPP=(F−E)+(H−G), based on signals E and F obtained respectivelyfrom two split photoelectric conversion sections of a first sub-beamphotodetector and on signals G and H obtained respectively from twosplit photoelectric conversion sections of a second sub-beamphotodetector; and a step of determining the differential push-pullsignal DPP=MPP−α×SPP (where α is a constant).